The present invention relates to a circuit for determining and evaluating a data signal on which a direct voltage portion is superposed, with a signal input, a signal processing means that has a filter and that forwards a part of the signal, and an evaluation means that compares the original signal from the signal input with the part of the signal forwarded by the signal processing means, and that determines the data signal. The present invention also concerns a method for determining and evaluating a data signal.
Direct voltages are often superposed on a signal, caused by offset voltages, tolerances, etc. In order to enable optimal use of the operational range of the members of a signal chain, it is necessary to compensate for these direct voltages, e.g. by determination of the direct voltage portion and subsequent subtraction. Direct voltage portions, also called zero-frequency quantities, are typically determined by low-pass filtering. An example of the necessity of such a circuit is synchronization in a TDMA system (time division multiple access). By such a system is meant multiple access in the time-division multiplex method. A transmitter transmits, in what is called a preamble, an alternating signal comprising a direct voltage portion of zero. Due to the non-ideal characteristics in the transmission chain, additional direct voltage portions occur at the output of the receiver, which have to be compensated for in order to achieve an optimal transmission quality. Since in a normal data transmission direct voltage portions or particularly low-frequency portions are possible, the direct voltage portion must be determined and then stored within the preamble, which has no direct voltage portion.
An example of such a TDMA system in which these problems occur is the new European cordless telephone standard DECT (Digital European Cordless Telephone).
In a known circuit, the direct voltage portion is determined by low-pass filtering. A compromise is thereby made between speed and precision with respect to the filter cut-off frequency and the time constant, which is inversely proportional thereto. The switching point at which switching over takes place from filtering to storage is thereby particularly problematic, because the system is not synchronized. One can either switch to storage as early as possible, at the expense of precision, or wait for the transition from the preamble to the normal data. Given the use of conventional linear filters, the latter can also ensue only with limited precision, since the switchover can only be recognized after a significant delay. In this case, the formation of mean values also takes place over a part of the data, whose mean value however must not be zero. The later the recognition of the end of the preamble and the switchover to storage takes place, the more faulty the mean value determination will be. In a known solution approach, the delay is solved by forming the mean value, not of the current signal, but of a delayed signal. Simple RC elements are used for this. The jump response of such filters always has a monotone characteristic, i.e., after the end of the preamble the signal systematically drifts in one direction, thus causing considerable halting errors under some circumstances. This error becomes smaller with higher-order filters, but does not disappear completely.